Method for heat treatment of fibrous mats

ABSTRACT

A technique is disclosed for heat treatment of fibrous mats, especially mats formed of mineral fibers such as glass and carrying a heat hardenable binder material, such as a thermosetting resin. The method disclosed provides for continuous feed of the mat through a curing or treating oven having a plurality of heat treatment zones. In one form of the method, the heat treatment in at least one zone is effected by circulation of a heated gas, such as air, through the mat in said zone, and the mat is also subjected to heat treatment in a localized area lying within said zone by passage of a second heated gas through the mat, the second heated gas having a pressure higher than that in the surrounding portions of said zone.

BACKGROUND AND STATEMENT OF OBJECTS

This application is a division of our application Ser. No. 912,482,filed June 6, 1978 now U.S. Pat. No. 4,263,007, Apr. 21, 1981.

In the production of fibrous insulating mats, especially mats formed ofglass or similar mineral fibers, it is customary to initially form themat by depositing the fibers on a perforated moving conveyor, usuallywith the aid of suction boxes provided under the flight of the conveyoron which the fibers are deposited. It is also customary to deposit afiber binder on the fibers either before or during the buildup of themat on the conveyor, such binder having adhesive characteristics andusually comprising a heat curable or hardenable material, such as athermosetting resin, for instance, a phenol formaldehyde resin sprayedon the fibers in a solution or a suspension in a volatile liquid, suchas water.

The layer or blanket of relatively loose fibers on the collectingconveyor is thereafter customarily delivered to what is commonlyreferred to as a mat curing oven through which the mat is fed byadditional perforated conveyor means, frequently comprising a pair ofendless conveyors having adjacent flights presented toward each other inspaced relation and serving to determine the thickness of the mat to beformed. Such a mat may be more or less dense, depending upon the extentof compression applied by the pair of conveyors in the mat curing oven.

During passage of the mat through the oven, the mat is subjected to heattreatment to effect curing of the fiber binder and thereby to effectstabilization of the mat at the desired thickness.

For the purpose of effecting the curing of the binder, varioustechniques have been employed; but quite commonly, the techniqueincludes passage of heated air through the mat, for which purposecirculation boxes or manifolds are arranged in pairs at opposite sidesof the feed path of the mat through the curing oven, such ovens quitecommonly including several such pairs of circulation boxes, withprovision for establishing different temperature conditions sequentiallythrough the series of pairs, so as to regulate the curing temperatureapplied at different zones in the path of the mat through the curingoven.

It is a principal object of the present invention to provide not onlyfor the heating to effect curing of the binder by the primary heatingsystem in the general manner heretofore contemplated, but in addition,the invention contemplates employment of a second independent heatingsystem comprising at least one pair of manifolds of relatively smallsize operating in relatively small localized areas at opposite sides ofthe path of the mat, this pair of manifolds serving to pass through themat a heated gas having a pressure and temperature sufficiently high toraise the temperature of the core portion of the mat to a higher valuethan that attained in the core portion in the areas surrounding saidlocalized areas. Moreover, the heated gas of this "secondary" bindercuring system is preferably passed through the mat in a localized arealocated in the mid or downstream portion of the feed path so that thesurface layers of the mat have already been cured and stabilized by theprimary heating system. This initial stabilization of the surface layersof the mat makes possible the use of relatively high pressure in thesecondary heating system without disrupting the fibers of the mat.

Although the method of the invention is adaptable to the curing of awide variety of mats and fibrous blankets, for reasons noted just above,the invention is especially advantageous in the curing of binder inrelatively dense mats, because the pressure and temperature conditionsemployed in the secondary heating system of the present inventionpromote rapid penetration of the heat into the interior of even quitedense and thick fibrous products; and since the secondary high pressureair is applied after the surface layers of the mat have been stabilized,this rapid penetration is accomplished without disruption of the fibers.

In a typical installation in which the primary heat curing systeminvolves the use of pairs of hot air circulation boxes arranged insequence along the feed path through the oven, the method of theinvention contemplates, as a secondary heat curing system, theintroduction of at least one pair of hot air circulation manifoldshaving relatively small localized areas lying within the zone or area ofone of the pairs of boxes of the primary system. In this installation,the method of the invention contemplates that the heated air of thesecondary system operating in the localized area have a pressure higherthan that of the air employed in the primary system. When employed inthis configuration, the primary air circulation system serves not onlyto supply heat needed for the curing of the binder, but in addition, itserves also as a means for preventing escape or loss into the atmosphereof air leaking from the secondary system which operates at higherpressure.

By the employment of both primary and secondary systems, and by theemployment of a higher pressure in the secondary system, the rapidpenetration of the heat into the interior of the mat being cured in thelocalized area of the secondary manifolds, is highly effective inexpediting attainment of the binder curing temperature in the interioror core portion of the mat; and it is an object of the invention toprovide for rapid attainment of a binder curing temperature sufficientlyhigh to initiate exothermic reaction of the binder resin. The attainmentof such an exothermic temperature will result in continuance of thebinder curing, even if succeeding zones of the curing oven are notmaintained at the same elevated temperature. Therefore, in the overallcuring operation, the use of the secondary high pressure system in thelocalized downstream curing areas effects an overall economy of thetotal fuel expended to accomplish the curing.

As disclosed and claimed in our application above identified, provisionis made for novel structural arrangements for introducing the highpressure manifolds of the secondary system in the localized areas of thecirculation boxes of the primary system, these structural arrangementsproviding for minimization of shortcircuiting and leakage and alsoproviding automatically for yielding of some of the shielding elementswithout breakage thereof, in the event of buildup of resin or otherdeposits on the conveyors serving to carry the mat through the curingoven.

BRIEF DESCRIPTION OF DRAWINGS

How the foregoing and other objects and advantages are attained willappear more fully from the following description referring to theaccompanying drawings, in which

FIGS. 1a and 1b, taken together, illustrate a longitudinal sectionalview through a mat curing oven used according to the present inventionembodying a sequence of six pairs of hot air circulation boxes providingthe primary system for the heat treatment or curing, and further asecondary system embodying two pairs of localized high pressuremanifolds, one pair being disposed in each of the last two of theprimary or low pressure circulation boxes;

FIG. 1c is a fragmentary view similar to a portion of FIG. 1b butillustrating an alternative embodiment in which two pairs of highpressure or secondary circulation boxes are enclosed in one of the pairsof the low pressure circulation boxes;

FIG. 2 is a transverse sectional view on an enlarged scale taken throughone of the pairs of primary hot air circulation boxes, as indicated bythe section line 2--2 applied to FIG. 1a;

FIG. 3 is a fragmentary longitudinal sectional view on the scale of FIG.2, illustrating one of the pairs of primary or low pressure circulationboxes having a pair of secondary or high pressure manifolds disposedtherein;

FIG. 4 is a view on the same scale as FIGS. 2 and 3 but illustrating atransverse section through a pair of secondary or high pressurecirculation manifolds, this view being taken as indicated by the sectionline 4--4 on FIG. 1b; and

FIG. 5 is a fragmentary view on a smaller scale than FIG. 4 butillustrating a modification of the high pressure air circulation system.

DETAILED DESCRIPTION OF THE DRAWINGS

In the drawings, the reference numeral 6 indicates the mat curing ovenin general, the oven having enclosing structures or walls within whichthe conveyor equipment and the hot air circulation systems are arranged.

As seen in FIGS. 1a and 1b, in the lower portion of the oven, rotativesupporting elements or rollers 7--7 are provided for mounting the lowerendless conveyor, the upper and lower flights of which are indicated inFIGS. 1a and 1b only by dot-dash lines, these conveyor flights appearingin greater detail at 8a and 8b in FIGS. 2, 3 and 4. As seen in FIGS. 1aand 1b, rotative supports or rollers 9--9 are also provided for theupper conveyor, which is indicated in FIGS. 1a and 1b only by dot-dashlines; but the lower and upper flights of which appear in greater detailat 10a and 10b in FIGS. 2, 3 and 4. Each of the conveyors is made up ofa multiplicity of links which are pivotally interconnected and whichcarry rollers 12 adapted to ride on the tracks indicated at 13. Thelinks are generally indicated at 11. Conveyors of this type are driventhrough the mounting rollers.

The rollers 9 and also the tracks 13 for the upper conveyor are mountedupon a frame structure 14 made up of longitudinal and transverse membersinterconnected so as to provide for adjustable positioning of the upperconveyor with respect to the lower conveyor. This adjustment may beeffected by screw jacks indicated at 15 in a manner well understood inthis art and forming no part of the present invention per se.

By virtue of the adjustability of the upper conveyor, the space betweenthe conveyor flights 8a and 10a, which are the runs presented toward thefibrous mat, may be altered in order to establish the desired density orthickness of the product being made.

At the upstream or input end of the oven, a conveyor, indicateddiagrammatically at 16, is provided, the conveyor here illustratedrepresenting a perforated conveyor such as commonly employed for thecollection of fibers to form a mat or blanket. Suction boxes, such asshown at 17, may be employed for assisting the collection of the fibersand for maintaining them in position on the conveyor. Suction fans 17aare connected with the suction boxes. The fibrous blanket carried bythis conveyor 16 is delivered to the sizing rolls R1-R2, which arepreferably adjustable in order to regulate the thickness of the matbeing introduced into the oven; and after delivery of the partiallysized mat from the rolls R1-R2, the mat enters between the conveyorflights in the curing oven.

Interiorly of the oven, the primary or low pressure air circulationboxes are provided in pairs. In the embodiment illustrated in FIGS. 1aand 1b, six such low pressure pairs of boxes appear, the zones orregions of these pairs being generally indicated by the letters A, B, C,D, E and F. These pairs of boxes are generally rectangular and areidentified by reference numerals 18 and 19. The boxes are closed on allsides except for the side presented toward the conveyor flights 8a or10a. Each box 18 is mounted on fixed structure below the upper flight 8aof the lower conveyor; and each box 19 is mounted on the verticallyadjustable framing 14 for the upper conveyor, so that the upper boxesmove with the upper conveyor when its position is adjusted.

Each box is also provided with an opening communicating with a duct foreither supply or exhaust of the treatment gases, such openings beingindicated at 20. The supply or inlet and the discharge or exhaustopenings are respectively marked with plus and minus symbols - "+" or"-". It will be noted that in the first pair of boxes A, the supplyopening 20 is arranged in the lower box 18 toward the upstream end ofthe box, with respect to the direction of feed of the product throughthe oven; and the exhaust opening is arranged in the upper box 19 ofthis pair, near the downstream end.

The same general pattern is repeated in the boxes of the second pairindicated at B. In the third pair of boxes, indicated at C, the inletopening is in the upper box 19 at the upstream end and the exhaustopening is in the lower box of the pair toward the downstream end.

In the zone indicated at D for the fourth pair of boxes, the pattern ofinlet and outlet openings is the same as for pairs A and B. In the pairof boxes E, the arrangement of inlet and outlet conforms with thatmentioned above in connection with zone C; and in the pair F, thearrangement shown conforms with that of the boxes D. It is to beunderstood that these relationships may be altered in order to vary themanner in which the curing is effected; and different flow conditionsmay be employed in connection with products of different types,thickness and/or densities, as is known in this art. In addition, asmaller or a larger number of circulation boxes and treatment zones maybe utilized, according to the nature of the product being made. Stillfurther, the flow through certain boxes may be shut off if desired.

Before considering the structure and operation of the high pressure aircirculation system contemplated by the present invention, attention iscalled to the fact that the overall enclosure of the oven 6 is providedwith a gas exhaust system including the ducts 21, and the exhaust fan22, the latter delivering gases removed from the interior of the oveninto and through an appropriate precipitator 23 for separation ofsuspended solids. The walls of the oven 6, in effect, comprise a hoodsurrounding the interior components of the oven including the heated gascirculation boxes described above, and the manifolds which are describedhereinafter; and the leakage which occurs is withdrawn from the ovenenclosure by the exhaust system just described.

FIG. 2 illustrates on an enlarged scale a transverse section through thelow pressure boxes of zone C. Here it will be seen that the gas supplyline 24 is connected with the upper or supply box 19 and that theexhaust duct 25 is connected with the lower exhaust circulation box 18.Vanes 19a serve to distribute the incoming gas over the width of theconveyor and thus over the width of the mat being treated. The gasesdischarged through the connection 25 are delivered to a heater 26 withwhich a burner 27 is associated, and these gases are drawn through theheater by the fan 28 and delivered by the fan into the supply duct 24.This gas heating and circulation system may be employed for more thanone of the pairs of low pressure boxes, or if desired, separatecirculation systems may be used.

To accommodate vertical motion of the upper conveyor and the partsmounted therewith, the supply duct 24 extends through an oversizedopening 29 in the wall of the oven, and a flexible closure bellows 30may be used to substantially seal the joint between the supply duct andthe wall of the oven. In addition, the duct 24 is provided with a slipjoint 24a to accommodate the vertical adjustment.

In considering the high pressure gas circulation system, attention isfirst directed to certain features of construction of the conveyors. Asabove noted, these conveyors are made up of links 11 which are pivottedto each other in an endless loop, one such loop being provided for eachconveyor. The individual links (see for example FIGS. 2 and 3) extendacross the width of the conveyor and have rollers 12 associatedtherewith at each edge of the conveyor, as appears from FIG. 2. Eachlink has a base plate 31 which is apertured at intervals across thewidth of the conveyor (as clearly appears in FIG. 2), and is providedwith projecting ribs or flanges 32 forming transverse passages extendedthrough the links for the flow of the gases from the low pressure supplyboxes (described above) or high pressure supply manifolds (describedherebelow), through the mat carried by the conveyors and then throughthe apertures and passages in the links of the other conveyor and intothe exhaust boxes or manifolds.

As shown in the embodiment of FIGS. 1a and 1b, a high pressure manifoldsystem HP1 is associated with the pair of low pressure boxes 18 and 19in zone E, this high pressure system and the pair of low pressure boxes18 and 19 being illustrated in enlarged longitudinal section in FIG. 3and in enlarged transverse section in FIG. 4. From FIGS. 1b and 3, itwill be seen that the high pressure manifold system is substantiallysmaller than the low pressure boxes 18 and 19 and further that the highpressure manifold system lies within the low pressure boxes 18 and 19.The high pressure supply manifold is indicated at 33; and fromcomparison of FIGS. 3 and 4, it will be noted that this manifold extendsacross the width of the conveyor, above the flight 10a, and of the matbeing treated, but is of relatively short dimension in a directionupstream and downstream of the feed path of the mat. A supply duct 34 isconnected with the high pressure supply manifold, this duct passingthrough an oversized opening 35 in the wall of the oven and the openingbeing closed by a flexible bellows seal 36. Duct 34 has a slip joint 34ato accommodate vertical motion. In the interior of the high pressuresupply manifold, vanes 37 are provided to insure distribution of thehigh pressure gases over the width of the conveyor. A high pressureexhaust manifold 38 is provided below the flight 8a of the lowerconveyor and the exhaust manifold is connected with the duct 39 in orderto discharge the high pressure gases after they have passed through themat being treated. The duct 39 delivers the withdrawn gases to a heater40 having a burner 41 from which the gases are withdrawn by the fan 42which recirculates the gases to the supply duct 34.

As in the low pressure system, the high pressure duct and circulationsystem, including the heater 40 and the fan 42, may be employed for morethan one high pressure system; or if desired, separate heaters and fansmay be utilized for different high pressure systems.

Although distributing vanes 37 are provided in the supply manifold 33,these are not needed in and are preferably omitted from the exhaustmanifold 38.

FIG. 5 schematically illustrates an alternative form of heater which maybe employed for heating the high pressure gases in the circulationsystem. Here, a heat exchanger diagrammatically indicated at 43, isintroduced into the exhaust manifold 39 in advance of passage of thegases through the fan or blower 42. FIG. 5 also indicates an alternativearrangement of the high pressure supply and exhaust manifolds 33 and 38;in this illustration, the supply manifold 33 being located below the matbeing treated and the exhaust manifold 38 being located above the mat.

Because of the employment of relatively high pressure gases in the highpressure system, it is of importance to minimize gas leakage, and thisrequires provision of special sealing devices, an example of suchdevices being particularly illustrated in FIG. 3. Here it will be seenthat at each side of the upper or supply manifold 33, a supportingstructure 44 is provided, this structure serving to mount a pair of wallelements 45, one located at each side of the manifold 33. Each of thesewall elements is pivotally mounted as indicated at 46, so that the wallelement may be swung or displaced upwardly away from the upper surfaceof the conveyor flight 10a. At a point opposite to the pivot 46, thewall element 45 is provided with a flange cooperating with a stop orabutment 47 which serves to limit downward movement of the wall elementand thus prevents contact of the wall element with the upper surface ofthe conveyor flight 10a. Each of these wall elements 45 is oftrough-like configuration, being extended across the entire width of theconveyor; and it is contemplated that these elements have a flat lowersurface and that they be mounted in close proximity to the upper surfaceof the conveyor flight 10a, thereby providing sealing action preventingany substantial lateral flow or leakage of the high pressure gas beingused in the high pressure manifold system. In a typical installation, inthe normal operating position of each wall element 45, the element willbe spaced from the upper surface of the flight 10a a distance of theorder of a few millimeters, for instance, from about 3 to 5 mm.

These displaceable sealing wall elements are provided so that thesealing elements may be normally positioned much closer to the conveyorthan would be possible if they were fixed in position. Displacement awayfrom the conveyor will readily occur in the event of the accumulation onthe conveyor of irregular deposits of resin or fibers, as tends to occurfrom time to time in the operation of such equipment. Since the elements45 are automatically displaceable, if a lump or deposit is encountered,no damage to the equipment will occur, even when the wall elements aremounted for normal operation very close to the surface of the conveyor.

The elements 45, located above the conveyor, function automaticallyunder the action of gravity to return to the position in close proximityto the conveyor, after being displaced by any deposit of resin orfibrous material. The width of the flat bottom surface of each of thetrough-shaped elements 45 is preferably at least as great as any twoadjacent passages between ribs 32 of the conveyor links 11, so that thedesired sealing function will be performed, regardless of the relativeposition of the ribs 32 with respect to the wall elements 45 in adirection along the path of movement in the conveyor.

Similar displaceable wall elements 48 are associated with the flight 8aof the lower conveyor, these wall elements being arranged for downwarddisplacement away from the lower surface of the conveyor flight 8a andbeing urged upwardly by springs 49. Similar pivots and limiting stopsare provided for the lower elements 48, but the lower elements beingdisplaceable downwardly under the action of obstructions encountered asthe conveyor flight passes the high pressure system, springs instead ofgravity are relied upon to return the wall elements 48 to their normaloperating position.

Each of the wall elements 45,45 and 48,48 is provided with an inclinedsurface, such as indicated at 50, at the upstream side of the element,in order to facilitate the displacement action under the influence offoreign bodies carried by the conveyors.

From FIGS. 1b, 3 and 4, it will be noted that in the high pressuremanifold system HP1, the supply manifold 33 is located above theconveyor and in the upper circulation box 19 of the low pressure zone Ewith which the low pressure gas supply connection communicates, thedischarge manifold of this high pressure system HP1 being located in thedischarge box 18 below the conveyor in the low pressure zone E.

By reference to FIG. 1b, it will also be seen that in the low pressurezone F, the high pressure system HP2 is inverted with relation to thearrangement shown in zone E. Thus, in FIG. 1b, the high pressure supplymanifold 33 is located below the mat in the low pressure supply box 18of zone F and the high pressure exhaust manifold 38 is located above themat in the low pressure exhaust box 19 of zone F.

In the alternative arrangement of FIG. 1c, two high pressure circulationsystems are shown as mounted within a single pair of low pressure boxes18 and 19. Thus, in this alternative embodiment, the high pressuresupply manifolds 33a and 33b are located in the low pressure box 19above the mat in side-by-side relation with an intervening displaceablesealing wall element such as the sealing elements described above inconnection with FIG. 3; and the cooperating high pressure exhaustmanifolds 38a and 38b are mounted within the low pressure exhaust box 18below the mat, with a displaceable wall element lying between the twohigh pressure exhaust manifolds of the type described above inconnection with FIG. 3. Outboard displaceable wall elements are alsoassociated with the high pressure manifold systems of FIG. 1c in themanner which will now be understood.

Although the high pressure air circulation systems contemplatedaccording to the present invention may be employed in association withany of the treatment zones, A to F, it is particularly advantageous toemploy such high pressure circulation systems in association with thelow pressure circulation boxes downstream of about the mid region of thefeed path and preferably at least 2/3 of the length of the feed pathfrom the entrance end of the oven. Thus, in accordance with onepreferred embodiment appearing in FIGS. 1a and 1b, two high pressurecirculation systems are indicated in general at HP1 and HP2, these beinglocated respectively within the low pressure zones E and F, being thelast two in the embodiment of FIGS. 1a and 1b.

Where two high pressure systems are incorporated in a single pair of lowpressure boxes, it is preferred to arrange the two high pressure supplymanifolds at the same side of the mat, and preferably within the lowpressure supply box, because this will minimize leakage problems, withconsequent loss of heat.

Operating Conditions

As will be understood, the operating conditions will vary in accordancewith a number of factors, including the thickness and density of the matbeing formed, the composition and characteristics of the binder beingused, and also the amount of binder employed. However, some generalguidelines, with regard to the operating conditions, are presentedherebelow.

First, it is contemplated that the low pressure circulation establishedby the circulation boxes 18 and 19, in the zones A to F inclusive,should include some zones in which the gases pass upwardly through themat, and some zones in which the gases pass downwardly through the mat.Also, it is contemplated that the gases circulated through the boxes 18and 19 in different zones may be at different temperatures, dependingupon the characteristics of the mat and the binder used, as is alreadyknown in the operation of mat curing ovens having multiple zones oftreatment. An appropriate temperature range for the gas supplied tocirculation boxes 18 and 19 is from about 150° C. to about 300° C., whenemploying common types of fiber binders, such as phenol formaldehydebinders.

The pressure conditions established may also vary, and these pressureconditions may be measured in various ways. The pressure in the supplybox and the pressure in the exhaust box will, of course, vary because ofthe drop in pressure incident to passage of the gas through the mat. Intypical operating conditions, the pressure in the supply box of the lowpressure systems may be of the order of from about 5 to 30 mm of water.

With regard to the high pressure circulation systems, it is contemplatedthat where more than one such system is utilized, for instance, in theconfiguration illustrated in FIG. 1b where one high pressure system HP1is located in low pressure zone E and another high pressure system HP2is located in low pressure zone F, it is contemplated that one of thesehigh pressure systems should be arranged to pass the treatment gasthrough the mat in one direction and the other high pressure system bearranged to pass the gas through the mat in the opposite direction.Thus, as indicated by the arrow in FIG. 1b, the high pressure system HP1is shown as delivering the gas downwardly and the high pressure systemHP2 is shown by the arrow as delivering the gas upwardly. This willserve to maintain substantial uniformity of the treatment throughout thethickness of the mat.

With regard to the pressure and temperature employed in the highpressure systems, it is further pointed out that some benefit may beachieved by the employment of both low pressure and high pressuresystems in combination in the same curing oven, even if the temperatureof the high pressure system is not higher than or even lower than thetemperature in the low pressure system. The reason for this is becausethe high pressure will cause more rapid and effective penetration of theheat to the interior of the mat than is the case with the low pressuresystem.

The high pressure system may be operated over a substantial range; butin general, should be at least several times, preferably at least 10 to20 times, the pressure of the low pressure system. For example, thepressure in the supply manifolds of the high pressure systems may beupwards of about 300 to 600 mm of water.

In a typical case where the temperature of the air in the low pressuresystems is from about 150° C. to about 300° C., the temperature in thehigh pressure systems may desirably be from about 200° C. to about 350°C.

In a typical installation, the high pressure systems may have adischarge flow of about 5,000 Nm³ /h, when the discharge flow of the lowpressure circulation system is about 30,000 Nm³ /h. The high pressuregases are concentrated in the localized relatively small areas ascompared with the low pressure gases, and those localized areas in atypical case may comprise about 10% of the area of the treatment zonesestablished by the low pressure boxes.

The temperatures and pressures will also vary depending upon the speedof advancement of the mat being formed and on the number of treatmentzones in the mat curing oven. The use, according to the invention, ofboth high and low pressure air circulation systems, is particularlyeffective from several standpoints, including the fact that for givenbinder curing effect, this may be accomplished in fewer treatment zonesand with a substantially shorter overall length of the curing oven. Thisis due to the fact that the high pressure systems are particularlyeffective in bringing the interior portions of the mat up to curingtemperature in a short time. It is also advantageous that the highpressure systems will rapidly bring the temperature of the binder to thelevel where exothermic reaction will occur even in the core portion ofthe mat; and this temperature will then be maintained more readily, evenbeyond the localized area of the high temperature manifolds.

We claim:
 1. A method for heat treating a moving fibrous mat carrying aheat hardenable fiber binder in order to effect hardening of the binderand binding of the fibers to produce a mat having substantially uniformbinding throughout the thickness thereof, comprising:(a) feeding thefibrous mat in a feed path through a heat treating zone of an ovenhaving an entrance and an exit and in which the mat is heated to atemperature having a hardening effect on said binder, and (b) passing aheated gas through the mat in a localized area smaller than said zone ofthe oven and extended across the mat in said zone, said heated gas beingat a temperature and pressure providing for heating of the interiorportion of the mat in said localized area to a temperature higher thanthe temperature established by the heating of the mat in said heattreating zone of the oven in a region adjoining and upstream of saidlocalized area, thereby effecting hardening of the binder in theinterior portion of the mat.
 2. The method of claim 1 wherein theheating of the mat in said adjoining region referred to step (b) iseffected by passing heating gas through the mat at a pressure lower thanthat of the gas passed through the mat in said localized area.
 3. Themethod of claim 2 wherein the temperature of the heated gas passedthrough the localized area referred to in step (b) is from about 150° C.to about 350° C. and in which the temperature of the heated gas passedthrough said adjoining region is from about 150° C. to about 300° C. 4.The method of claim 2 or claim 3 wherein the pressure of the heated gasin said localized area is at least several times that of the heated gaspassed through said adjoining region.
 5. The method of claim 2 or claim3 wherein the pressure of the heated gas in said localized area is atleast 10 to 20 times that of the heated gas passed through saidadjoining region.
 6. The method of claim 2 or claim 3 wherein thepressure of the heated gas passed through the mat in said adjoiningregion is from 5 to 30 mm of water.
 7. The method of claim 1 whereinsaid localized area referred to in step (b) is located in the feed pathof the mat through the heat treating zone downstream of at least the midpoint between said entrance and exit of the treating zone in the oven.8. A method for heat treating a moving fibrous mat carrying a heathardenable fiber binder in order to effect hardening of the binder andbinding of the fibers to produce a mat having substantially uniformbinding throughout the thickness thereof, comprising:(a) feeding thefibrous mat through a heat treating zone of an oven having an entranceand an exit and in which a first heated gas is passed through the matfrom one side face to the opposite side face of the mat, and (b) passinga second heated gas through a localized area of the mat lying withinsaid zone, which area is smaller than the total area of said zone, (c)the temperatures of both of said heated gases being sufficiently high tohave a hardening effect on said binder, and the pressure of said secondheated gas being higher than the pressure of said first heated gas toprovide for increased hardening effect on said binder in said localizedarea.